1. Field of the Invention
The present invention relates to an imaging lens to be mounted on an imaging apparatus using an imaging device, such as a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor).
2. Description of the Related Art
Recently, size reduction and pixel-density increase have advanced conspicuously for the imaging apparatuses, such as of CCDs and MOSs. This in turn requires an imager proper and a lens for mount thereon, that are smaller in size but higher in performance. Meanwhile, in order to cope with the imaging device having pixels with density, there is also a need to provide a telecentricity, i.e. characteristic to place the main rays of light incident upon the imaging device at an angle nearly parallel with the optical axis (incident angle upon the image plane nearly zero relative to the normal line to the image plane). Conventionally, there is known an imaging lens, in a three-lens arrangement, to be mounted on a camera-equipped cellular phone or the like. JP-A-2003-322792 describes an imaging lens having so-called an inter-stop structure formed by totally three lenses and arranged with an aperture stop between the first lens and the second lens. However, with an inter-stop structure, if attempted to reduce the total length of the lens system, the angle is increased of the main rays of light upon the image plane, thus resulting in a worse telecentricity. For this reason, it can be considered to arrange the aperture stop in a position closest to the object in order to ensure the telecentricity. JP-A-2004-219807, JP-A-2004-240063 and JP-A-2004-317743 describe image lenses each having a three-lens structure whose aperture stop is arranged closest to the object. In the three-lens-structured imaging lens arranged with an aperture stop closest to the object, the first lens frequently is made convex at its surface closer to the image surface, particularly made convex at both surfaces.
However, in the case of arranging the three-lens-structured imaging lens with an aperture stop in a position closest to the object, it is advantageous to reduce the total length and secure the telecentricity. Nevertheless, the sensitivity in manufacture is raised to readily cause variations on the image surface in the case there is a lens positional deviation in the manufacture. Particularly, where the first lens is convex in form at both surfaces, the sensitivity-in-manufacture is ready to rise. Meanwhile, recently, a development is desired of an imaging lens well relieved of chromatic aberration, in order to cope with increasing pixel density.